Carbon nanofiber means a fibrous material, which includes 90% or more of carbon components and has a diameter of less than 1 μm, and may be manufactured by a method such as electrospinning, chemical vapor deposition, and laser evaporation. Since carbon nanofiber grown by the method forms sp, sp2, and sp3 hybrid bonds, and has flexibility, high strength, and high elasticity together with heat resistance, chemical stability, high electrical conductivity, mechanical strength, and a high specific surface area, it is possible to apply the carbon nanofiber to various fields from advanced materials, such as electrode materials of secondary batteries and fuel cells, catalytic agents, sensor materials, and electromagnetic wave shielding materials, to life science.
Carbon-based materials used as an anode material of a lithium secondary battery have a maximum charge capacity of 372 mAh/g, may reversibly intercalate/deintercalate only one lithium ion per 6 carbon atoms, and thus have a limitation in capacity. As a material capable of complementing the limitation, silicon is used, and silicon is capable of intercalating and deintercalating lithium due to the crystal structure thereof, and has a maximum charge capacity of about 4,000 mAh/g, which is 10 times larger than that of carbon-based materials. However, silicon has problems in that electrical conductivity deteriorates, and during the intercalation and deintercalation of lithium ions, the volume of the crystal structure is changed by 300% or more and the change in particle size is so great that the cycle does not reversibly proceed.
In order to suppress the change in volume of silicon and stabilize the anode structure, numerous inventions for improving the lifespan and capacity of an electrode have been conducted using nanoparticlulation of silicon and materials such as a silicon-metal alloy, a silicon-non-metal alloy, and a silicon-carbon composite.